The present invention relates to a method and apparatus for applying a coating material to a moving web of paper, and in particular to a coating method and apparatus of the trailing blade type which includes an internal leveling blade upstream of the trailing blade.
Conventional coaters of the trailing blade type include means for applying coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible coater blade located on the trailing side of the applicator, which serves to doctor or level the applied coating. In general, an excess of coating material is applied on the web, and the coater blade then meters or removes the excess while uniformly spreading the coating onto the web surface.
In recent years, it has become desirable to produce papers having a minimum amount of coating. To achieve low coat weights with conventional trailing blade equipment, it is necessary to increase the pressure of the coater blade against the web, which results in a high rate of wear of the blade and necessitates more frequent replacement of the blade. High blade pressure also increases the possibility of web breakage and streaking caused by foreign particles being caught between the blade and web.
Conventional coaters inherently employ a relatively long dwell or soak time, which is the time interval between the initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersible materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive web coats without intervening drying because the successive coats tend to migrate into and contaminate the previous coat.
To overcome the disadvantages of the aforementioned applicators in applying lightweight coatings on paper, there has been developed a short dwell time applicator, as disclosed in U.S. Pat. No. 4,250,211, assigned to the assignee of the present invention. In that applicator, means is provided with an upper outlet slot through which coating material is introduced into a relatively narrow application zone for being applied in excess on a web carried therethrough. The coating material in the application zone and as applied to the web is maintained under pressure, and the speed of the web is adjusted for a relatively short dwell time. A flexible coater blade doctors the web at the downstream end of the application zone, thereby removing excess material from the web and at the same time uniformly spreading the material on the web. In consequence of the short dwell time and the pressurized application of coating material on the web, an appropriate yet lightweight amount of coating material may be applied on the web without the need for high blade pressures.
A difficulty occasionally encountered in use of conventional applicators as well as those of the type disclosed in said U.S. Pat. No. 4,250,211 is that, depending upon the nature of the paper and coating material and the amount of material to be applied on the web, the coating may fail to properly penetrate and fill voids and valleys in the surface of the web. When this occurs, the quality of the coating is impaired.